For a number of years the coating industry has been engaged in substantial developmental programs in the quest for procedures which would allow the coating of substrates at high production rates with the coating being cured to a tack-free condition at a speed commensurate with the contiguous processing steps. The industry has desired to eliminate the volatile solvents required in many of the well-known coating processes because of potential hazards or because of the cost of equipment to handle the evolved solvent vapors. In addition, the industry has been seeking coating formulations which would produce coatings which were durable and which would permit substantial additional processing of the workpiece, such as metal forming operations where the substrate is metal strip for container bodies, blanks and closures.
Epoxy coating formulations have long been recognized as affording desirable properties in the finished coating, especially the toughness to withstand further processing. However, the problem has remained to develop a low cost epoxy coating formulation which would combine the desired rheological properties for the coating application with both reasonable pot life and rapid curing in the production line.
In Watt U.S. Pat. No. 3,794,576 granted Feb. 26, 1974, there are described desirable epoxy formulations which combine the desired rheological properties with suitable pot life and rapid curing, by incorporation of a photoinitiator and at least about 15 percent by weight of an epoxidic ester having two epoxycycloalkyl groups. However, such esters materially increase the cost of the formulation as compared with the more conventional epoxide prepolymer materials.
Since the disclosure of Watt, a number of patents and publications have appeared proposing various photoinitiators for the epoxy formulations which could replace the diazonium catalysts specifically described in the Watt patent. Among these are the onium catalysts disclosed in Barton U.S. Pat. No. 4,090,936 granted May 23, 1978; Crivello U.S. Pat. Nos. 4,069,055 granted Jan. 17, 1978 and 4,058,401 granted Nov. 15, 1977.
Although it has long been known that heat will accelerate the reaction rate of polymerization following irradiation to effect complete curing, generally in accordance with the Arrhenius equation, and has long been suggested to augment the irradiation treatment (see, for example, the aforementioned Watt patent at Column 6, lines 1-16), such heating has also been recognized to introduce other problems in the form of production requirements and possible adverse effects upon the polymer properties since low molecular weight polymers may result. Some researchers have evaluated various factors affecting cure rate in such irradiated epoxide formulations and have proposed substantially elevated temperatures to increase the cure rate while avoiding volatilization of the monomer (See, Crivello et al, "Triaryl Sulfonium Salts: A New Class of Photoinitiators for Cationic Polymerization," JOURNAL OF RADIATION CURING, Volume 5, pages 2, 10-11, January 1978).
In the copending application of Joseph M. Guarnery and William R. Watt, Ser. No. 20,515, filed Mar. 14, 1979, entitled RAPID CURING OF EPOXY RESIN COATING COMPOSITIONS BY COMBINATION OF PHOTOINITATION AND CONTROLLED HEAT APPLICATION, there is disclosed a process in which the coating is maintained at a temperature of 50.degree.-90.degree. C. for a period of 0.5-2 minutes following initiation of exposure to radiation to effect rapid curing to a tack-free condition.
However, the problem has remained to provide relatively economical coating compositions which would provide the benefits of epoxy polymers but at lower cost. Inclusion of inert fillers has tended to undesirably effect polymer properties, and inclusion of most polymeric materials as fillers results in stability or rheological problems.
Accordingly, it is an object of the present invention to provide a novel photoinitiable epoxy composition which is relatively economical and which can produce a tack-free surface condition rapidly for use on high speed production equipment.
It is also an object to provide such a composition which is relatively stable in rheological properties and which generates a coating which exhibits highly desirable mechanical properties.
Another object is to provide a novel coating process using such compositions which is adaptable to a wide variety of high speed coating lines and which does not require extensive or expensive equipment.
A further object is to provide such a method in which the polymeric materials of the coating composition are integrated into a strong, adherent polymeric film.